Performance evaluation of personalized ventilation personalized exhaust (PV PE) system in air conditioned healthcare settings 4

UFAD ventilation inlet Velocity inlet; flow rate=20l/s ; I = 20%; D=0.2 Room air outlet Pressure outlet; Gauge pressure=0 Pa Personalized exhaust outlet Pressure outlet 4.2.4 Parametric

Chapter 4: Preliminary Studies 4.1 Introduction

The objective of the preliminary studies is to evaluate and verify the feasibility of the novelPersonalised Ventilation - Personalised Exhaust (PV-PE) devices There are quite a few variations influencing the design of PV-PE system, such as the position of

PE, the pressure of PE, background ventilation type, PV ATDs type etc Before building the actual experimental facilities, the preliminary research involving a few parametric variation studies was envisaged to provide ideas to make the experimentalset-up more optimal.The preliminary research comprised two pilot studies Pilot study I was aimed to evaluate the feasibility of PV-PE system using Computational Fluid Dynamics (CFD) simulation It was found that the PV-PE system was able to divert the PV fresh air profile and exhaust the exhaled contaminated air before it mixed with the room air In Pilot study II, three different types of PE devices were developed and compared using a CFD model The results show that the top-PE and shoulder-PE could have a better performance in practice

4.2 Pilot study I - Feasibility of the novel PV- PE system

4.2.1 Research Methodology

CFD simulation was used to evaluate parametric variations and to find the feasibility

of the novel PV- PE system

4.2.2 Geometry and Grid

Two human beings sitting at two sides of a desk in a consultation room were simulated as numerical manikins using CFD The dimension of the simulated consultation room is 4 m (length) x 3 m (width) x 2.6 m (height) The two manikins were sitting face to face with a 2 m distance between each other One represented an Infected Manikin (Infected Person) that acts as a source of contaminated exhaled air and the other manikin was assumed to be a Healthy Manikin (Healthy Person) that inhales the contaminated air and acts as a sink The geometry of the computer simulated thermal manikin used in the pilot study is obtained from physical thermal manikin by using a three-dimensional laser scanning technique (Gao& Niu, 2006) It

is a real and accurate representation of a nude seated female occupant with the surface

area of 1.57 m2 A pair of newly conceptualized localized exhaust devices, termed the Chair Personalised Exhaust (PE) devices, was placed at the upper part of the chair and just behind the human head, with a dimension of 0.08 m x 0.08 m

Room geometry was divided into two sections: lower part of the room enclosing the two manikins, with 2,863,045 unstructured tetrahedral cells around the manikin, and the remaining upper room volume with 1,496,069 cells There are two layers of

uniform boundary layer cells placed around the manikin’s body with size of 0.0022 m Enhanced wall treatment was applied in the simulation As shown in Figure 4.1, the value Y+ was approximately in the range of 0.5-1.8 for most part across the body surface and around 3 for small part at face area This is acceptable since in practice,

an up to 4-5 is considered acceptable as it is still inside the viscous sub-layer

Figure 4.1 Contours of wall Y+

4.2.3 Turbulence model and boundary conditions

The indoor air flow patterns was simulated using standard k-epsilon model, which has been used in previous numerical research (Gao & Niu, 2004; Pantelic, 2010) to simulate indoor air flow based on the assumption that this model is capable of simulating convective heat transfer of buoyancy-driven air flow as long as a reasonable value of Y+ is achieved Energy equation was activated Continuity equation and momentum equations were solved to obtain velocity distribution Species transportation equation was activated to study the personalized air distribution and exhaled air spread The SIMPLE algorithm is used to couple the pressure and +

y

velocity fields Considering the convection and diffusion, second order upwind was used to solve the momentum and energy equations PRESTO was used for pressure The room air was defined as incompressible air since the speed of room air flow was insignificant compared to the speed of sound of the fluid medium As shown in Figure4.2, a four way ceiling supply air diffuser, demonstrated to be able to predict the air flow pattern accurately (Cheong et al, 2001), was modelled for the MV inlet since it is the most commonly used diffuser for MV In addition, Under-Floor Air Distribution (UFAD) system was also considered The UFAD outlet with a diameter

of 200 mm is modelled in the simulation Both the axial-velocity and velocity were assigned in boundary conditions in order to represent the swirling flow from a floor-mounted circular diffuser Detailed boundary conditions used in this study are listed in Table 4.1

tangential-Figure 4.2 Models for the four way air diffuser

Table 4.1: Detailed boundary conditions in pilot simulation study

Turbulence model Standard k–epsilon model

Number of cells used in the Fluent

model

3,763,604 cells Mixing ventilation inlet Velocity inlet; T = 23 °C; I = 10%

UFAD ventilation inlet Velocity inlet; flow rate=20l/s ; I = 20%;

D=0.2 Room air outlet Pressure outlet; Gauge pressure=0 Pa

Personalized exhaust outlet Pressure outlet

4.2.4 Parametric Variation Studies

Two sets of CFD simulation were carried out The first set was conducted to evaluate the influence on PEE by adding a Personalized Exhaust (PE) working together with

PV In this simulation, both the manikins keep inhaling at a velocity of 8.4 l/min Two different kinds of PV air terminal devices (ATD) were simulated: Round Moveable Panel (RMP) and Vertical Desk Grill (VDG) Two different background air supply systems were simulated: ceiling supply Mixing Ventilation (MV) system (Figure 4.3) and Under-Floor Air Distribution (UFAD) system (Figure 4.4) Detailed conditions used in this study are listed in Table 4.2

Figure 4.3 Configuration of the simulated office with mixing ventilation (1-MV four- way inlet; inlet d=200 mm; 2-MV outlet 500x500 mm; 3-RMPd=120 mm;

4-PE 80x80 mm; 5-VDG 220x20 mm; 6-Numerical manikin)

Figure 4.4 Configuration of the simulated office with UFAD ventilation (1- UFAD inlet d=200 mm; 2- UFAD outlet 500x500 mm; 3-RMP d=120 mm; 4-PE

80x80 mm; 5-VDG 220x20 mm; 6-Numerical manikin)

Table 4.2: Detailed ventilation combinations studied in Set 1 simulation

In order to evaluate the ability of personalized exhaust (PE) system in preventing the spread of contaminated air exhaled by infected people, another set of CFD simulation was conducted The dimensions of the simulated room were the same One manikin (Healthy Manikin) keeps inhaling at a velocity of 8.4 l/min through mouth (20 × 10 mm) and the other manikin (Infected manikin) keeps exhaling at the same velocity through mouth (20 × 10 mm) Tracer gas was introduced in the exhaled air Based on Set I simulation results, the combination of MV and Vertical Desk Grill (VDG) has the lowest PEE Set II simulations chose this worst case combination as the ventilation combination to do further studies Detailed conditions used in this study are listed in Table 4.3

Table 4.3: Detailed ventilation combinations studied in set 2 simulations

Ventilat

ion

Mixing Ventilation +VDG

4.2.6 Results and Discussion - Ability to change the PV air direction and profile

The first objective of the pilot study was to examine the advantages and feasibilities

of the novel PV-PE system, which are mainly focused on the ability to pull the PV fresh air by the chair integrated PE devices CFD simulation performed in Set I studies compared the PV air streamlines in each PV-background combination with PE and without PE Figure 4.5 to Figure 4.8 show that there are great differences between each case The streamlines in Figures 4.5 through 4.8 use the “Unique option” which gives each streamline a different color along its whole length, and can be used to track individual streamlines through the domain The purpose of the figures is to show how the PE diverts the PV air So only streamline from PV is shown The streamlines from UFAD or MV inlet are not shown in these figures When PV ATD (either RMP or VDG) was coupled with a PE device, almost 100% PV air went through the seated occupant towards the PE device, compared with only a small portion reaching the seated human being without PE device After hitting the surface of the manikin in the face region, a fraction of the air flow may move up towards the ceiling exhaust grill This fraction depends largely on the gauge pressure of the PE

(a)

(b)

(c)

(a) No PE device (b) Gauge pressure at -10Pa (c) Gauge pressure at -50Pa

Figure 4.5 Streamlines under the combination of VDG and MV

(a)

(b)

(c)

(a) No PE device (b) Gauge pressure at -30Pa (c) Gauge pressure at -50Pa

Figure 4.6 Streamlines under the combination of RMP and MV

(a)

(b)

(c)

(a) No PE device (b) Gauge pressure at -30Pa (c) Gauge pressure at -50Pa

Figure 4.7 Streamlines under the combination of RMP and UFAD

(a)

(b)

(c)

(a) No PE device (b) Gauge pressure at -10Pa (c) Gauge pressure at -50Pa

Figure 4.8 Streamlines under the combination of VDG and UFAD

4.2.7 Results and Discussion - Personalized Exposure Effectiveness (PEE)

The second objective of the first pilot study was to evaluate the improvement of the inhaled air quality Hence, the PEE is calculated and compared

Figures 4.9 and 4.10 display the changes of Personalized Exposure Effectiveness (PEE) with the change of gauge pressure of PE From the two figures, it can be deduced that the percentage of personalized air in inhaled air has a different trend

with mixing ventilation and with UFAD ventilation In the following analysis, the profile of PEE will be discussed separately together with different background ventilation types

For MV systems, PEE concentration distributions increase when air flow rates increase When PE is added, PEE remains unchanged at low flow rate level (8 l/s) and decreases with the decrease of gauge pressure of PE at higher flow rate level (12 l/s and 16 l/s) The largest reduction is around 20% when gauge pressure is -50 Pa compared with no PE devices This is because the suction force of PE devices tends to divert the PV air in front of the face instead of reaching the breathing zone

For UFAD systems, PEE concentration distributions increase when air flow rates increase as well For the same air flow rate, RMP has a better performance than VDG

in terms of PEE PEE shows an obvious increasing trend by adding a PE device For both RMP and VDG, PE device enlarged the concentration of personalized air in the breathing zone For RMP, the increased PEE is only applicable for higher flow rates (12 l/s and 16 l/s) The PEE remains unchanged for low flow rate, such as 8l/s For VDG, the increase of PEE is obvious and the increased amount is larger when PV supply flow rate is at 8 l/s

Figure 4.9 Comparison of PEE at different gauge pressures of PE with Mixing

Ventilation

Gauge pressure of PE

Mixing ventilation

Figure 4.10 Comparison of PEE at different gauge pressures of PE with UFAD

ventilation

4.2.8 Results and Discussion - Intake Fraction

As stated before, Intake Fraction can be calculated by inhaled tracer gas concentration for the Healthy Manikin over the exhaled tracer gas concentration from the Infected Manikin For all the cases in Set II studies, the exhaled air was marked as ‘exhaled air’

in FLUENT software The concentrations of exhaled air at the mouth of the Infected Manikin for all the cases are the same as shown in Figure 4.11

Figure 4.11 Concentrations of exhaled air at the mouth of the Infected Manikin

The inhaled tracer gas concentrations at the mouth of Healthy Manikin when PV air flow rate was set at 8 l/s are shown in Figure 4.12

Gauge pressure of PE

UFAD ventilation

(a) No PE device

(b) Gauge pressure at -10Pa

(c) Gauge pressure at -30 Pa

(d) Gauge pressure at -50 Pa

(e) Gauge pressure at -80 Pa

Figure 4.12 Concentrations of exhaled air at the mouth of Healthy Manikin with

a PV flow rate of 8 l/s

The inhaled tracer gas concentrations at the mouth of Healthy Manikin when PV air flow rate was set at 12 l/s are shown in Figure 4.13

(a) No PE device

(b) Gauge pressure at -10 Pa

(c) Gauge pressure at -30 Pa

(a) No PE device

(b) Gauge pressure at -10 Pa

(c) Gauge pressure at -30 Pa

(d) Gauge pressure at -50 Pa

(e) Gauge pressure at -80 Pa

Figure 4.14 Concentrations of exhaled air at the mouth of Healthy Manikin with

a PV flow rate of 16 l/s

Figure4.15 illustrates the iF for all the cases in Table 4.3 for at the mouth of the Healthy Manikin Generally, iF reduces with the increase of PV supply rate, which indicates that PV has the potential to protect people from inhaling pollutant air Furthermore, for PE with gauge pressure of -80 Pascal, iF of all the three supply flow rates was about one order of magnitude lower than without PE In terms of protecting the healthy manikin from pollutants exhaled by the polluting manikin, adding a PE device is much more effective than increasing the PV flow rate This is because PE devices are able to exhaust the exhaled air directly before it mixes with the room air,

as shown in Figure4.16

Figure 4.15 Comparison of iF at different Gauge pressure of PE

Figure 4.16 Air streamlines showing exhaled air from Infected Manikin

4.3 Pilot study II - Evaluation of different Personalized Exhaust devices

The first pilot study has examined the feasibility of Personalized Ventilation - Personalized Exhaust (PV-PE) system and supports the idea of supplying more fresh air to a person as well as to exhaust the exhaled contaminated air directly around the Infected Person before it mixes with the room air in hospitals and healthcare centres Thus, the location and design parameter of the PE devices can be further explored since it plays a major role in the distribution of air around the human body

In pilot study II, three different PE devices were developed, simulated and compared:

a chair-PE, the same as used in Pilot study I; a top-PE, which is a round device above the human head; a shoulder-PE, which are two local exhaust devices installed at the chair, just above the shoulder level The PV air terminal devices chosen in this study are VDG and Desk-top PV (DPV) The performance of the PV-PE system in regard to occupants’ inhaled air quality and the transmission of exhaled aerosols between two occupants was studied and investigated numerically by computational fluid dynamics

50 mm above the manikin’s head, Chair-PE and Shoulder-PE had a dimension of 80

mm x 80 mm Table 4.4 shows the details of the simulated conditions in this study

PV flow rate was set at 4 l/s or 8 l/s